Ghassem and others,
Before you consider using FEMA 273, you might want to look at a FEMA 343
Report "Case Studies: An Assessment of the NEHRP Guidelines (FEMA 273) for
the Seismic Rehabilitation of Buildings." Unfortunately, this important
report, which is based on the over two million dollar Building Seismic Safety
Council Case Studies Project and written by Andrew Merovich, is still in
press. When it is available, you can obtain a free copy from the FEMA
Publications, Telephone 800-480-2520.
It is not clear why this important FEMA 343 Report is not readily available
and is still "In Press", except to comment that FEMA 343 raises some very
important issues about the technical adequacy of the some of the FEMA 273
Guidelines. This FEMA 343 Report was delivered, camera ready, to FEMA on
October 1, 1999.
When you obtain your copy of FEMA 343, turn to page 89, Section 6.2,
Technical Adequacy, Item T1, which addresses the problems with how FEMA 273
inadequately attempts to deal with overturning. In Section 6.2, Technical
Adequacy, Item T1 it states: "The treatment of overturning in the linear
procedures produces results that are much more severe than observations of
past building performance imply are necessary. The Guidelines provides a
sidebar that can be used to adjust overturning demands to levels consistent
with those of new construction designed by current code procedures. At a
minimum, the sidebar should be modified to include a reduction in earthquake
demand consistent with the removal of coefficients C1, C2, and C3. This
modification should result in overturning demands that are consistent with
current codes for new constructions, BUT IT DOES NOT ADDRESS THE RESULTING
INCONSISTENCY IN DEMAND FORCES ABOVE THE FOUNDATION INTERFACE AND THOSE
REDUCED FORCES BELOW IT." (Emphasis added.)
In other words, following the FEMA 273 Guidelines, the forces at the base of
the superstructure connections to the top of the foundation and not in
equilibrium with the forces at the foundation-soil interface.
The primary question then is how does a design professional design the
foundation system when the forces at the top of the foundation are not in
equilibrium with the forces at the foundation-soil interface?
Hopefully, the ASCE/FEMA 273 and the ASCE/FEMA 310 Standards, which are now
under preparation, by the ASCE Standards Committee on the Seismic
Rehabilitation of Buildings, will provide adequate guidance on how to address
the problem of overturning following the FEMA 273 Guidelines. For additional
information concerning ASCE/FEMA 273 and ASCE/FEMA 310 Standards log onto the
Degenkolb Engineers web site: www.degenkolb.com/ascefema273.
Frank E. McClure January 7, 2000